Conventionally, the power steering system is known, and according to the power steering system disclosed in Japanese patent publication No. 50-33584, the steering effort or torque applied to a steering wheel is assisted by an output torque from an electric motor. The gain of the system for the signal obtained by detecting the steering torque applied to the steering wheel by the vehicle operator is varied according to the vehicle speed and the road condition, and the output torque of the assisting electric motor is accordingly changed so that the optimum actuating torque which actually steers the front wheels may be obtained at all times.
When the vehicle is abruptly subjected to a strong crosswind while travelling a straight course, the vehicle may deviate from the intended straight path of travel. In such a case, it is necessary to apply a counteracting force to the steerable wheels so as to oppose such interferences. However, according to such a conventional power steering system, when the vehicle has started deviating from the intended path as a result of crosswind, because the electric motor can produce an actuating torque only when the vehicle operator applies a steering torque to the steering wheel, the electric motor cannot produce any actuating torque unless the vehicle operator intentionally takes a corrective action.
To control the deviation of the vehicle from the intended straight path, the vehicle operator must apply a corrective steering torque to the steering wheel. However, according to the conventional power steering system, the required steering torque increases as the lateral acceleration and the yaw rate of the vehicle increase, and, accordingly, the steering torque required for counteracting the deviation of the vehicle due to external interferences increases as the lateral acceleration and the yaw rate of the vehicle increase.
Furthermore, the conventional power steering system requires a very small steering torque for its operation, but the vehicle operator receives very little feedback from the steering wheel with regard to the behavior of the vehicle. Therefore, the vehicle operator must depend on his vision and bodily sense of acceleration in assessing the condition of the vehicle when it is subjected to external interferences. As a result, the counter-action by the vehicle operator tends to be delayed, and, therefore, tends to be excessive.
To overcome such problems, it is conceivable to detect the irregular behavior of the vehicle due to external interferences from the yaw rate of the vehicle, and produce a force counteracting the irregular behavior of the vehicle by using an assisting motor so that the irregular behavior of the vehicle may be controlled.
However, according to such a control system based on the use of an electric motor, because the system cannot determine whether the detected yaw rate is due to an intentional steering maneuver or due to external interferences, the electric motor always produces a steering torque which tends to control the yaw rate or which tends to restore the vehicle back to the straight ahead path of travel irrespective of the cause of the yawing movement. As a result, the steering reaction which the electric motor produces may become excessive when the vehicle is making a normal turn. If the control parameters are modified so that the steering reaction may be appropriate at the time of normal steering maneuver, the vehicle may not be sufficiently protected from external interferences.